An internal combustion engine generally includes one or more reciprocating pistons operably connected to a crankshaft by a connecting rod to deliver energy therefrom. Such engines can operate on a 4-stroke cycle with sequentially timed phases of intake, compression, power, and exhaust, as well as 2-stroke-cycle. In a spark ignited engine a mixture of fuel and air is compressed by the piston within the closed confines of the cylinder. Near the end of the compression phase ignition occurs either by compression heating or by an ignition device to form hot burning gas which expands and drives the piston to create power.
In a diesel engine air is similarly compressed during the compression phase. Near the end of the compression phase, fuel is injected into the cylinder where it will mix with the compressed air and spontaneously ignite to form hot burning gas which, during the expansion phase of the cycle, expands against the movable piston and thereby creates power.
The last several years has seen an increasing emphasis on designing for improved fuel economy and efficiency, reduced emissions, a greater service life, and an increased power output per unit engine size.
The power of engines having similar combustion systems can be compared on the basis of their relative air flows. Conventionally there are two approaches to obtaining high air flow rates through a given engine volume. The first is the application of turbochargers to raise the density of the air entering the cylinders. The second is typified by the Wankel rotary piston engine in which a major geometric change achieves large "cylinders" in a small engine volume. The highest power density levels are achieved by combining these methods. The rotary piston engine is unsuited geometrically to withstand high cylinder pressures and to use high compression ratios so the fuel economy is poor and the potential for using turbochargers to boost the inlet air to high density is limited to levels below conventional engines.
Conventionally reciprocating engines use large bore/stroke ratio when optimized for power and a smaller bore/stroke ratio when optimized for economy. This stems from the importance of providing enough valve area for good breathing for power and a compact combustion chamber with high compression for economy. By using two stage compression and expansion processes it is possible to achieve induction in a large bore/stroke cylinder and combustion in a more compact space and so to achieve a better combination of high power and high efficiency.
It is well known that dilution of charge by recirculating exhaust gas can have a beneficial influence on flame temperature and the level of NO.sub.x emissions from an engine. Conventionally the exhaust gas subtracts from the space available for a fresh charge of air and so this method is inappropriate for high power levels.
Accordingly what is needed is an improved internal combustion engine which is (a) compact because it has large "cylinders" for induction relative to its volume, (b) efficient because it can use high compression ratios with a compact combustion space and, (c) low on NO.sub.x emissions because it achieves high levels of exhaust retention without loss of power.